System and method for analysis of fibre reinforced composites

ABSTRACT

A system for analyzing fiber reinforced composite including: an ultrasonic transmitter configured to provide ultra-sonic pulses to the fiber reinforced composite; an ultrasonic receiver configured to receive ultrasonic signal data related to the ultrasonic pulses; a filter module configured to filter the ultrasonic signal data; a signal processing module configured to process the filtered ultrasonic signal data; an analysis module configured to analyze the processed ultrasonic signal data by: calculating a characteristic value based on the ultrasonic signal data; comparing the characteristic value to a baseline established for the characteristic value; and determining a percentage of design strength based on the comparison; and an output module configured to output the percentage of design strength.

RELATED APPLICATIONS

This application is a continuation of U.S. patent application Ser. No.15/996,786 filed Jun. 4, 2018 which is a continuation of U.S. patentapplication Ser. No. 14/882,570 filed Oct. 14, 2015, which claims thebenefit of U.S. Provisional Patent Application No. 62/063,568 filed Oct.14, 2014, the content of which is incorporated herein by reference.

FIELD

The application relates to a system and method for analysis of fiberreinforced composites.

BACKGROUND

Ultrasonic readings taken from fiber reinforced composites, such asplastics, can be related to the elastic modulus, and hence the strengthof the material. Research results dating back to the 1960's have shownthat stressing fiberglass reinforced polymers (FRP) may result indecreasing the modulus of the material—thus reducing its strength. Inservice, the stresses applied to FRP typically have the same effect.

An early patent application regarding use of ultrasound for testingmaterials was made in 1940 by Dr. Floyd Firestone at the University ofMichigan. This early patent, and most subsequent work using ultrasoundidentified that invisible inhomogeneities within materials could bedetected. For most of the 74 years since this patent application, thefocus of ultrasonic testing has been on metals. In the case ofultrasound, a pressure pulse is applied to a metal material andinhomogeneities are detected when a feature blocks some of the path ofthe ultrasounds—features that are parallel to the path direction aregenerally not detected.

Use of fiber reinforced composites for structural applications has beenpursued since the 1930's, and has seen significant changes in thepolymers and fibers available. With the growth of commercial aircraftstarting in the 1960's, many investigations were conducted into use ofultrasound to detect flaws and defects in composites. Because many fiberreinforced composites are made in layers, interfaces between layersoften interrupt the path of the pressure pulses and show as features orpossible defects for most ultrasonic techniques. Ultrasound is generallyconsidered to be the most common non-destructive technology used forcomposite materials.

In the early 1960's, use of ultrasonic testing (UT) was already showingreliable results for finding flaws in metallic structures. One of thedesirable attributes of this technique is that reliable data could begenerated if only one side of the material under investigation wasaccessible. This meant that in addition to finding flaws or defects, thesame techniques could be used to produce thickness records of reasonableaccuracy. At the same time, use of composite materials such as glassreinforced thermoset plastics was being explored for a number ofstructural and corrosion-resistant applications. Starting in the mid1960's, researchers started to examine uses of ultrasound with thesefiber-reinforced composite materials.

In one study, ultrasonic pulses were applied to composites and theresponses were received using acousto-ultrasonic devices, thus mixingthe principles of ultrasound with acoustic emission testing. Thisprocess is generally referred to as “acousto-ultrasonic” because theforces applied to the specimen are from ultrasonic pulses, whereas foracoustic emission, the forces applied to the composite are frommechanical loads, such as pressures and weights. In both cases, theresponses are received in real time by acoustical equipment. This workshowed correlation between the attenuation of the signal transmittedthrough the full thickness of a laminate—across its layers—and itstensile strength parallel to its layers. This technique is the subjectof two American Society for Testing and Materials (ASTM) standards—ASTME 1495 Standard Guide for Acousto-Ultrasonic Assessment of Composite,Laminates and Bonded Joints² (ASTM E 1495) and ASTM E 1736 StandardPractice for Acousto-Ultrasonic Assessment of Filament Wound PressureVessels³ (ASTM E 1736).]

A method to employ these techniques is described in ASTM E 1736. In thisStandard Practice, it is recommended that initial readings be taken fromthe vessel to be monitored after calibration to a reference standard andbefore it is put into service. After the unit has been in service forsome time, the results of the initial readings are then compared toreadings taken after the unit has been in service. Changes that haveoccurred in the modulus of the composite from corrosion, decay ormechanical loads will appear as changes in the results of the scan.While there is a relationship between acousto-ultrasonic results and thepresence of detectable defects such as voids or delaminations andporosity, it is not certain as yet whether or not these defects are thecause of strength changes. Furthermore, in order to make use of anycorrelation, it is generally required that reference standards beavailable for each feature and condition that requires detection.

Many users of structural composites can report that the structuralcapacity of the composite has reduced while it has been in service.There have been numerous investigations into this phenomenon, includingproposed models of the causes of these changes.

Embodiments of the system and method described herein are intended toaddress at least one of the drawbacks of conventional systems andmethods.

SUMMARY

In a first aspect, the present disclosure provides a system foranalyzing fiber reinforced composite, the system including: anultrasonic transmitter configured to provide ultra-sonic pulses to thefiber reinforced composite; an ultrasonic receiver configured to receiveultrasonic signal data related to the ultrasonic pulses; a filter moduleconfigured to filter the ultrasonic signal data; a signal processingmodule configured to process the filtered ultrasonic signal data; ananalysis module configured to analyze the processed ultrasonic signaldata by: calculating a characteristic value based on the ultrasonicsignal data; comparing the characteristic value to a baselineestablished for the characteristic value; and determining a percentageof design strength based on the comparison; and an output moduleconfigured to output the percentage of design strength.

In a particular case, there is provided a memory component configured tostore ultrasonic signal data and baseline for characteristic values.

In another particular case, the filter module is configured to extractrelevant data from the ultrasonic signal data.

In still another particular case, the relevant data includes dataassociated with the material being tested and the extracted dataincludes data associated with the ultrasonic transmitter and theultrasonic receiver.

In yet another particular case, the relevant data includes a magnitudeof a reflection from the opposite surface of the fiber reinforcedcomposite.

In still yet another particular case, the output module is furtherconfigured to output a projection of a time remaining prior to apredetermined threshold value is reached.

In a particular case, the predetermined threshold value is a valuerelated to a replacement requirement.

In still another particular case, the output module is configured tooutput data related to a strength level of a bonding at joins of thefiber reinforced composite.

In another aspect, there is provided a method for analyzing fiberglassreinforced polymer, the method including: taking ultrasonic signal datafrom the fiber reinforced composite; receiving the ultrasonic signaldata, at an ultrasonic receiver; filtering the ultrasonic signal data,at a filter module; processing the filtered ultrasonic signal data, at asignal processing module; analyzing the processed ultrasonic signaldata, at an analysis module, wherein the analysis comprises: calculatinga characteristic value based on the ultrasonic signal data; comparingthe characteristic value to a baseline established for thecharacteristic value; and determining a percentage of design strengthbased on the comparison; and displaying the percentage of designstrength of the fiber reinforced composite, at an output module.

In a particular case, the method further includes storing the ultrasonicsignal data and baseline for characteristic values at a memorycomponent.

In another particular case, the filtering of the ultrasonic signal dataincludes extracting relevant data from the ultrasonic signal data.

In still another particular case, relevant data includes data associatedwith the material being tested and the extracted data includes dataassociated with the ultrasonic transmitter and the ultrasonic receiver.

In yet another particular case, the relevant data includes a magnitudeof a reflection from the opposite surface of the fiber reinforcedcomposite.

In still yet another particular case, the method includes displaying aprojection of a time remaining prior to a predetermined threshold valueis reached.

In another particular case, the predetermined threshold value is a valuerelated to a replacement requirement.

In yet another particular case, the method includes displaying datarelated to a strength level of a bonding at joins of the fiberreinforced composite.

Other aspects and features of the present disclosure will becomeapparent to those ordinarily skilled in the art upon review of thefollowing description of specific embodiments on conjunction with theaccompanying figures.

BRIEF DESCRIPTION OF THE DRAWINGS

Embodiment of the present disclosure will now be described by way ofexample only, with reference to the attached Figures.

FIG. 1 illustrates destructive test results from samples removed fromglass fiber reinforced tank shell;

FIG. 2 illustrates an embodiment of a system for analyzing fiberreinforced composites;

FIG. 3 illustrates an embodiment of a method for analyzing fiberreinforced composites;

FIG. 4 illustrates normalized strength percentage results according toan example experiment;

FIGS. 5A to 5D illustrates non-destructive parameters plotted againstdestructive test results;

FIG. 6 illustrates a conversion curve;

FIG. 7 illustrates an example A-scan;

FIG. 8 illustrates data extraction from the A-Scan of FIG. 7;

FIG. 9 illustrates velocity for samples that do not contain sand filler;

FIG. 10 illustrates velocity for samples that do contain sand filler;

FIG. 11 illustrates the velocity data and normal distribution forsamples not containing sand;

FIG. 12 is a histogram illustrating the difference to mean L_(tt);

FIGS. 13A and 13B illustrate sonic velocity results in an experimentusing an embodiment of the system and method described herein;

FIGS. 14A and 14B illustrate sonic velocity results of polymersincluding sand in an experiment using an embodiment of the system andmethod described herein;

FIGS. 15A and 15B illustrate flexural parameter results in an experimentusing an embodiment of the system and method described herein;

FIG. 16 illustrates stereography images for glass fiber constituentsafter burn-off, for fibers with an angle of −57°, fibers with an angleof +57°, and fibers with random orientation;

FIG. 17 illustrates raw and transformed reading from a case studydetailed herein;

FIG. 18 illustrates an FRC split ring testing schematic;

FIGS. 19A and 19B illustrate a sample after fracture and a loaddeflection curve of the sample;

FIGS. 20A and 20B illustrate a sample after fracture and a loaddeflection curve of the sample;

FIGS. 21A and 21B illustrate a comparison between average destructivetest modules and predicted modules from thickness (FIG. 21A) and transittime (FIG. 21B).

DETAILED DESCRIPTION

Conventional research has been successful in identifying that changes infiber reinforced composites (FRC) (such as fiber reinforced polymers,fiber reinforced plastics, fiberglass reinforced composites, fiberglassreinforced polymers, or the like) for example reduced strength orelastic modulus, occur over time. Reduction in structural capacity hasnot been universally associated with any change in defects that arenormally detected by ultrasonic methods, such as voids, delaminations orporosity within the composites. It is most common for defects anddiscontinuities to be widely dispersed and not identified as discreteflaws.

In some cases, ultrasonic pulses may be used to detect flaws or providedata related to fiber reinforced composites. Ultrasonic pulses can beapplied to materials in three main modes:

-   -   pulse-echo, where the pulse is applied to the surface by the        same transducer that receives reflected energy from within the        material,    -   thru-transmission, where the pulse is applied to one surface by        one transducer and the pulses that pass through the material are        received by a transducer placed on the opposite surface, and    -   pitch-catch, where the pulse is applied to the surface by one        transducer and another transducer on the same surface receives        reflected energy within the material.

Ultrasonic pulses can generally range in frequency from approximately100,000 Hertz (0.1 MHz) to beyond 20 MHz. When used with glassreinforced composite materials, signal losses in the material increasewith frequency, making the highest reasonable frequency approximately1.0 MHz. In the experiments detailed herein, a nominal ultrasoundfrequency of 500 kHz was used. In the experiments, attenuation valuesgenerally ranging from 0.3 to 2.0 decibels (dB)/mm (7.62 to 50.8 dB/in)were also used.

In ultrasonic testing, an energy pulse is applied to the face of amaterial by an ultrasonic device, for example, an actuator, atransducer, or the like. These pulses have a short wavelength whichtranslates into a wave frequency in the range listed above. Ultrasounduses two primary modes to travel through a material—longitudinal andtransverse waves. The experiments detailed herein used mainlylongitudinal waves and fiber reinforced composites.

Currently, the most common use of longitudinal waves in FRC is inthickness measurement of new FRC structures. Thickness measurements areusually made by following this process:

-   -   1. A reference standard is used which duplicates the material to        be measured with a known thickness so that the transit time of        the reflected signal can be used to determine the sonic velocity        through the reference standard    -   2. It is assumed that the sonic velocity through the material to        be measured is the same as the reference standard.    -   3. The transit time of ultrasonic pulses applied to the material        is converted into thickness.

Thickness testing generally does not use any other information containedin the returned ultrasound signal.

Flaws such as voids, porosity and planar defects that interrupt the pathof the ultrasonic wave through a fiber reinforced composite will appearin an ultrasonic A-Scan and can often be analyzed by a skilled analyst.This principle is used for evaluating composites in some applications,aerospace in particular.

Propagation of sound waves through a medium is affected by changes alongthe wave path. Examples of these changes could be, for example, foreignobjects, gaps or bubbles, changes in the structure of the material, orother changes. In the case of fiber reinforced composite materials, thestructure of the material may include some (and sometimes all) of thesechanges along any wave path. These generally show as attenuation of anysignal that passes through the material as well as visible indicationson the test instrument. For glass reinforced composites, normalvariations that occur because of materials and processes used may oftenbe cause for rejection if using the same criteria that have been adoptedfor metals.

Several researchers have reported experimental results showing acorrelation between the elastic modulus of FRC and acousto-ultrasonicresults. This includes correlation of changes in strength that hasoccurred from applied stresses and chemical permeation, corrosion, orattack with changes in ultrasonic response of the FRC. These earlyresearchers have successfully shown that acousto-ultrasonic methods canbe used to determine general changes in condition of compositelaminates.

It is important to note that the correlation does not mean that thevalue of the elastic modulus can be determined directly from theacousto-ultrasonic data. To determine the actual modulus it is generallynecessary to know the modulus value corresponding to anacousto-ultrasonic value on at least one point along the curve. However,with the appropriate criteria, it is believed that this information canbe used to determine whether a composite laminate is suitable for theloads to be applied in service conditions.

The central parameter measured by acousto-ultrasonic methods is termedthe “stress wave factor” (SWF), which may be determined by counting thenumber of pressure pulses greater than a predetermined threshold valuereceived by the sensor over a period of time, based on use of acalibration standard which came from the original, new composite. It isintended that the SWF from a composite can be used to determine thecondition of the composite.

The embodiments of the system and method detailed herein use the outputof ultrasonic flaw detection equipment to determine parameters that areused to calculate the condition of the composite. The output of theultrasonic equipment appears as a generally static Cartesian graph, withtime along the horizontal axis and overall magnitude on the verticalaxis. The graph displayed is referred to as an “A-scan” and it may beconfigured to show the time series of signals received from pulsesapplied to the material.

The method, according to an embodiment herein, used to determine thestate of the fiber reinforced composites, is configured to extractrelevant data from the raw A-scan after various filters have removeddata that is not associated with the actual material being tested,including data from within the transducer and instrument system. Thisdata is identified both as a result of statistical analysis and fromspecific ultrasonic readings included in the process. This filteredA-scan then represents the net response of the material to the forcefrom the ultrasonic pulse. One of the key parameters determined from thefiltered A-scan is the magnitude of the reflection from the oppositesurface, known herein as the Net Opposite Surface Reflection Magnitude(NOSRM). The method does not make use of calibration standards and therelevant data is obtained from the filtered A-scan.

FIG. 1 shows the results from tests of samples removed from a glassreinforced tank on two (2) occasions. The tank had been in servicestoring a corrosive liquid. The results shown are for the measuredthickness of the tank shell and the results of destructive testing. Theresults of the tests are shown as percentages as given by equation (1)below.

$\begin{matrix}{{{Percentage}\mspace{14mu} {of}\mspace{14mu} {Original}} = {\frac{{Current}\mspace{14mu} {Measurement}}{{Original}\mspace{14mu} {Measurement}} \times 100\%}} & (1)\end{matrix}$

From FIG. 1, it can be seen that during fourteen (14) years of service,the thickness of the laminate did not change appreciably but the modulusreduced by 40%. At some point in this decline, it is likely that thecomposite will no longer be able to support the required loads.

Two approaches are proposed for monitoring the condition of structuralcomposites—one is to use a baseline developed from specimens of new FRCcombined with theoretical work and the other is to use the results ofultrasonic readings taken from the new structure to be monitored and touse this as the baseline. Either approach is intended to addressdrawbacks of previous approaches.

Reliable performance of composites in structural applications,especially where remaining service life prediction is sought after,requires non-destructive methods that can verify structural propertiesincluding mechanical strength. Through the development of reliablenon-destructive methods, regular evaluations can be completed to monitorcondition. Such a system permits owners to avoid costly consequences(such as premature repair and replacement, confined space entry,environmental cleanup and lost opportunity) and capitalize on repeatableand reproducible information to manage repair and replacement scopeswithin budget cycles.

Developments in ultrasonic non-destructive evaluation of glassreinforced composites have generally been made using open-mold methodsand laminate thickness of 6 mm and greater. Emphasis is on monitoringthe condition of composites structures in service, using technology thathas been developed and is described herein. One purpose of thisdisclosure is to describe an objective system and methodology fordetermining baseline values for use in monitoring changes occurring tocomposite structures in service.

Reliable performance of composites in structural applications,especially where life prediction is desired, requires non-destructivemethods that can verify structural properties including mechanicalstrength. With availability of reliable non-destructive methods, regularevaluations can be completed to monitor condition.

At this writing, for industrial and civil applications there is not agenerally accepted non-destructive methodology to determine whether acomposite structure being put into service meets the designrequirements. Furthermore, for composite structures that have been inservice for some time, relevant non-destructive data is rarely availablefrom the new structure, which prevents comparisons related to changesthat have occurred and thus the determination of current suitability forservice.

First, consider a parallel situation where a steel structure is to beevaluated. For steel, structural capacity is generally related directlyto thickness. In this case, the original thickness is documented, say ona drawing or specification. Conventional non-destructive methods can beused to reliably determine the current thickness of the steel. It isreasonable to use the original documented thickness as the startingthickness, even if the actual thickness was different. From the startingthickness, the rate of thickness change can be determined and lifeprediction of the structure can be estimated, even when initialmeasurements might not be available.

FIG. 2 illustrates a system 100 for analyzing fiber reinforcedcomposites. An ultrasonic device 105 is configured to provideultra-sonic pulses to a fiber reinforced composites. The ultrasonicdevice 105 may be configured to receive readings, or ultrasonic signaldata, and transfer readings to an ultrasonic module 110. In other cases,the ultrasonic module 110 may be configured to receive the ultrasonicreadings directly.

The system 100 will generally include a filter module 115 configured tofilter the ultrasonic readings. A signal processing module 120 will alsogenerally be included and configured to process the ultrasonic readingsprior to the analysis of the readings. In some cases, the filter module115 and signal processing module 120 may be a single module.

The system 100 includes an analysis module 125 configured to analyze theultrasonic readings. The analysis module 125 is configured to processthe ultrasonic signal data, calculate a characteristic value based onthe processing of the ultrasonic signal data, compare the characteristicvalue to a baseline established for the characteristic value, anddetermine a percentage of design strength based on the comparison. Thenormal characteristic value reported by the calculations is thepercentage of the original flexural modulus of the composite. An outputmodule 140 may be configured to receive the comparison and percentage ofdesign strength and provide an output, for example a report, to adisplay 145.

A variety of outputs can be provided to provide information on thecondition of the composite. For example, a primary report can providedetails on the locations where ultrasonic readings were taken and mayinclude a report of how the condition, in particular the flexuralmodulus of the composite, has changed during service life of the fiberreinforced composites. Based on the time-rate-of-change, a projectionmay be made of the remaining time until a specific value will bereached, for example: a predetermined replacement value, a risk levelvalue or the like. Further outputs available may relate to the extent orstrength of bonding at joints and the amount of damage that has occurredto portions of the composite that have been exposed to chemicalenvironments.

A damage mechanism refers to changes that occur in a material that canlead to or predict failure. For fiber reinforced composites there arethree (3) major damage mechanisms. Damage to the resin or matrixdescribes damage that can occur and accumulate in the resin due toconditions imposed on the composite. This damage can take the forms ofmicrocracking and chemical changes to the resin. Damage to thereinforcement fibers can occur as chemical damage such as leachingelements from the glass or mechanical damage to the fiber, for exampletensile fracture. Damage to the resin-fiber interface is damage that canoccur to the bond between the fibers and the resin. Examples includechemical damage from corrosion or shear fracture of the bond. Damagemechanisms and failure modes, which are visible to the naked eye, aredescribed as they are important for ongoing evaluation of the structuralcapacity of FRC.

The system 100 is further configured to include a memory component 130configured to store data, such as, for example, baselines forcharacteristic values, thresholds, and the like. The system includes aprocessor 135, configured to execute the instructions or commands fromthe other modules.

FIG. 3 illustrates a method 200 for analyzing fiber reinforcedcomposites.

At 205, ultrasonic readings are taken from the composite material. At210, the ultrasonic module receives the readings related to detectingflaws in the material. At 215, the readings are digitized by theultrasonic module 110.

At 220, the readings or signals are processed and filtered using thesignal processing module 120 to convert all data to a consistent timeand magnitude scale and the filtering module 115 to remove extraneousdata created by external causes. At 225, data calculations are performedby the analysis module 125. The data calculations completed use thefiltered results to determine the actual transit time and attenuationand are combined to produce the output values and at 230, the outputmodule 140 provides a report or other output to the display 145.

Now consider a typical situation as shown in the case illustrated inFIG. 1, above, where thickness is not expected to change but the modulusdoes. In this case, non-destructive measurements similar toacousto-ultrasonic results described above are required to know thecurrent relative modulus value. In order to create a prediction of therate of modulus change, it will be necessary to have a starting value.For the purpose of this disclosure, the starting value, or “New” value,is the baseline value. It will be understood that other starting valuesmay be used as a baseline value.

1. Experimentation

The work described herein was performed to develop a comparison betweenparameters calculated from ultrasonic readings and standard destructivetest results for glass reinforced composite laminates.

1.1 Hypothesis

Ultrasonic data from a variety of glass reinforced composites can beused to establish a “universal” baseline parameter.

1.2 Experimental Method

To compare methods, samples of glass reinforced composites wereproduced, or removed from various structures, and tested both by theultrasonic methods described herein and by using standard destructivetests to determine the modulus of the material. A total of thirty-six(36) samples were used. Thirty percent (30%) of the samples were newlymade, and the remaining seventy percent (70%) had been in service for upto thirty (30) years.

The samples were constructed using open mold techniques and had varyingreinforcement content. Twenty-five (25) samples were made using filamentwinding and ten (10) used contact molding. Each sample was approximately300 mm×300 mm in size. Sample thicknesses ranged from eight (8) mm(0.315 inch) to forty-eight (48) mm (1.890 inches). Most of the sampleswere from cylindrical shells where the material properties in the hoopdirection were of most interest.

The samples used for these experiments were manufactured by twelve (12)different manufacturers using different methods and practices. For thesamples that were provided from structures exposed to corrosivesubstances, chemical attack and absorption was also different. It isexpected that differences among samples from these factors will alsointroduce random variation.

The designation of the destructive test used is ASTM D 790.

1.2.1 Non-Destructive Tests

The ultrasonic readings were taken by in the manner described below. Allreadings were pulse-echo readings using a 0.5 MHz transducer with avulcanized rubber delay line. For each sample, the average thickness wasmeasured using a caliper and recorded. At least 30 readings were takenover the surface of each sample.

The ultrasonic readings were then processed, via the filter module 115and the signal processing module 120 to identify the opposite surfacereflection and to calculate the value and total transit time of thereflected peak. Thickness was used to determine the average sonicvelocity for the reading.

The results of all readings for a sample were averaged.

For each sample the following non-destructive parameters werecalculated:

$\begin{matrix}{L = {{f({attenuation})} = \frac{NOSRM}{\left( {{Gross}\mspace{14mu} {Attenuation}\mspace{14mu} {by}\mspace{14mu} {the}\mspace{14mu} {Material}\mspace{14mu} {Tested}} \right)^{0.25}}}} & (2) \\{\mspace{79mu} {L_{t} = {L \times {thickness}}}} & (3) \\{\mspace{79mu} {L_{tt} = {L \times {transit}\mspace{14mu} {time}}}} & (4) \\{\mspace{79mu} {V = {2 \times \frac{thickness}{{transit}\mspace{14mu} {time}}}}} & (5) \\{\mspace{79mu} {{LV} = {L_{t} \times V}}} & (6)\end{matrix}$

The values calculated using (2), (3), (4), (5) and (6) were tabulated bysample.

1.2.2 Destructive Tests

For each of the samples, the procedure outlined below was followed:

-   -   1. Ultrasonic data and thickness measurements were collected        from the sample.    -   2. The sample was cut into test specimens in accordance with        ASTM D 790.    -   3. Where the lamination sequence of the sample was unknown, a        specimen was also cut for ignition loss analysis in accordance        with ASTM D 2584 and reinforcement analysis.    -   4. A third-party test laboratory completed the ASTM D 790 and        ASTM D 2584 (as applicable) testing and reported the results.    -   5. The third party laboratory returned the reinforcement from        the ASTM D 2584 specimen as it was removed from the furnace.    -   6. The ASTM D 2584 residue was used to determine the lamination        sequence.    -   7. The Design Flexural Modulus was modeled using lamination        analysis as described in ASME RTP-1.    -   8. The flexural modulus result obtained from the ASTM D 790 test        was normalized by dividing it by the design values from        lamination analysis modeling and termed “Normalized Strength        Percentage” as in equation (6).

$\begin{matrix}{{{Normalized}\mspace{14mu} {Strength}\mspace{14mu} {Percentage}} = {\frac{{ASTM}\mspace{14mu} D\mspace{14mu} 790\mspace{14mu} {Modulus}}{{Design}\mspace{14mu} {Flexural}\mspace{14mu} {Modulus}} \times 100\%}} & (7)\end{matrix}$

An example of the calculation process is shown below:

TABLE 1 Calculation Example Design ASTM D Flexural 790 Modulus ThicknessModulus Normalized in GPa in mm in GPa Strength (Msi) (in) (Msi)Percentage 15.36 (2.229) 48.03 (1.891) 9.25 (1.343) 60.2%

2. Results 2.1 Normalized Strength Percentage

The results of the normalized strength percentage calculations arepresented in FIG. 4. The data have been ordered from highest to lowest.Note that the values range from 43% to 132% of the calculated designvalue.

2.2 Non-Destructive Parameters

For all readings taken from each sample, the reflection of the appliedpulse from the opposite surface was selected. Where the opposite surfacereflection could not be identified, the reading was discarded.

The values listed in equations were calculated and averaged for thesample. The values of L_(t), L_(tt), V, and LV were then plotted withthe corresponding normalized strength percentage as shown in FIG. 5 (a)to (d), below. The scatter of the data points is believed to be largelydue to random variation introduced by differences among samples asdiscussed above.

Correlation coefficient and R-squared values for linear regressionbetween the normalized strength percentage and the calculated valueswere calculated and are shown in Table 2.

TABLE 2 Correlations with Normalized Strength Percentage Non-DestructiveParameter and Equation Number L_(t) L_(tt) V LV (3) (4) (5) (6)Correlation Coefficient 0.871 0.827 0.365 0.898 R-squared 0.759 0.68330.133 0.806

From Table 2, the best correlation and linear regression resultscorrespond with the LV value determined by equation 6. These data areshown in FIG. 5(d). Calculation of this value requires the magnitude andtransit time of the opposite surface reflection from the ultrasonicreadings as well as the thickness of the composite. If the thickness ofthe composite is unknown, the parameter L_(tt) is the alternative,although with lower correlation.

2.3 Sonic Velocity Considerations

FIG. 5(c) shows the data where sonic velocity was calculated accordingto equation (5). The correlation coefficient for this data (Table 2)shows poor correlation with Strength Percentage. As well, correlationbetween sonic velocity and L_(tt), which does not include knowledge ofmaterial thickness, was poor at 0.365. This discussion shows that sonicvelocity generally should not be used as an indicator of compositestrength in the applications considered here and it cannot be modeledusing the ultrasonic parameters discussed here.

From examination of the data in FIG. 5(c), it appears that sonicvelocity may converge to a narrower range at higher Strength Percentage.From this, at best, one can only be expected to provide a possible rangeof thickness.

2.4 Baseline Calculation

The intent of this paper is to identify values that could be used asBaseline, or starting, values for composites being evaluated formechanical strength. The results described above show that two (2)parameters calculated from ultrasonic readings can be used to determinethe strength of a glass reinforced composite as a percentage of thevalue determined from lamination analysis. From the data that wasconsidered in this paper, solving the linear regression curves for thevalue that produces 100% will yield the baseline values.

For the two (2) parameters selected above, the Baseline values are shownin Table 3. For composites where the original parameters are unknown,assuming that the original strength was 100% of the calculated valuewould allow use of these values to determine the starting point. Theratio of current values to the baseline value can be used with theconversion curve shown in FIG. 4 to provide the Strength Percentage.

Note in FIG. 4 that the slope of the curve changes where the percentageof design strength is about 45%. This is done to take the strengthpercentage to 0% when the parameter is 0. However, at this point, thechange in slope is not supported by data, since no samples tested inthis paper have Normalized Strength Percentage less than 43%.

TABLE 3 Baseline Values Parameter Baseline Value L_(tt) 4.2606 LV 19.085

In the case where ultrasonic readings can be taken from a compositebefore it is put into service, the results of the readings can be usedwith the baseline values above and FIG. 4 to provide the starting pointvalue for future evaluations.

This disclosure has shown that non-destructive ultrasonic methods canshow strong correlation with the bulk elastic modulus of a wide range offiber reinforced composites. Conventionally, it has been shown that thereduction in bulk modulus appears to occur at thereinforcement-to-matrix interface, inferring that this modulus reductionis independent of the type of fiber reinforcement. Damage of theseinterfaces has been produced in the laboratory by several means,including absorption of liquids and mechanical stresses—the sameconditions that many structural composites must accommodate.

The correlation of parameters determined from the ultrasonic readingshas been demonstrated, above. When ultrasonic readings are taken from areinforced composite and processed by the method described herein,changes in the values of the parameters calculated using equations 4 or6 are used to determine the current modulus value using FIG. 4.

In practical application, the strain at failure is generally relativelyconstant for a composite. Reductions in bulk modulus will see the strainincrease at a constant stress—therefore showing reduced strength. Thus,as the bulk modulus declines due to various environmental and loadingconditions, the strain within the composite will increase. Failure willoccur when the strain has increased to the failure level without anychange in the load or application conditions.

The ultrasonic methods described herein can be applied to compositestructures for the purpose of monitoring changes in the bulk modulus ofthe material. When the Strength Percentage is determined for an as-builtcomposite—or LV or L_(tt) are determined directly from the as-builtstructure—then changes in the bulk modulus can be determined fromultrasonic readings. Over time, the changes in bulk modulus can then beincorporated into risk assessment to determine whether the resultingstrains are acceptable and to project remaining service life.

This is similar in outcome to thickness testing of metallic structuresfor the same purpose. In this case, the elastic modulus remains constantbut the stress level increases due to reduction in area.

Most composite structures presently in use did not have the new StrengthPercentage, LV or L_(tt) determined when they were new. As such, they donot have a known starting point from which to determine the rate ofchange or to make projections. In these cases, use of the baselinevalues listed in Table 3 is intended to allow initial risk assessment tobe conducted. Because the testing is non-destructive, the riskassessment can be updated frequently with minimal effect on thestructure.

A further item to discuss is that the parameters calculated did notrequire explicit calibration standards. Only the samples provided wereused with no outside reference standards. As well, different ultrasonicequipment—flaw detectors and transducers—were used for some of thesamples. This makes it possible to use ultrasonic methods for strengthevaluation of existing composite structures without access tocalibration standards.

The experimentation has been conducted on a wide variety of compositesmade using open-mold methods with epoxy vinyl ester matrices and glassreinforcements. Similar results have been achieved with otherconstituent materials and construction methods, as well as with othermanufacturing and exposure effects.

The following conclusions are expected based on the foregoing:

-   -   Ultrasonic readings can provide reliable information about the        current strength of reinforced polymers;    -   Changes in composite strength determined using ultrasonic        methods can be used for life prediction;    -   Composite strength values can be provided without using        calibration standards; Sonic velocity does not provide reliable        correlation with composite strength; and    -   The calculation methods used in can generally be applied to at        least open-mold composites.

A further experiment was conducted on new glass reinforced thermoset,which were obtained from various sources. The specimens were of varyingsizes and compositions. Testing was completed on 46 specimens with anaverage of 39 readings per specimen. All readings were taken withoutoverlap. Forty-one (41) of the specimens were composed only of glassfiber reinforcement in a polyester or vinyl ester matrix. Five of thespecimens included various amounts of sand filler.

The readings were taken using Olympus Epoch 1000 ultrasonic flawdetector with M2008, 0.5 MHz transducer.

3.1 Hypothesis

The hypothesis of this work is that quantitative non-destructiveparameters can be obtained from new glass reinforced composites todemonstrate consistent properties.

3.2 Ultrasonic Testing Background

In this experiment, an ultrasonic transducer was used to apply a short,small displacement pulse to the surface of the composite. The pulse timewas approximately 1 microsecond, corresponding to a frequency of 0.5megahertz.

The pulse transits the material though the thickness until it reflectsfrom the free opposite surface and returns to the origin. Along thepath, the energy of the pulse is attenuated by the material and variousfeatures within the composite. Between pulse applications, thetransducer is used to receive the reflected signals. The reflectedsignals are then accumulated according to the time after the pulse wasapplied.

The output available from the ultrasonic equipment are displayed on anx-y display, with time along the horizontal axis and the averagedmagnitude of the received signal displayed vertically. This output isreferred to as an “A-scan”. An example is shown in FIG. 7, where thesignal reflected from the opposite surface is labeled.

3.3 Sonic Velocity

The study that was completed was to take ultrasonic readings from onesurface of the specimen, spaced adjacent to each other. The thickness ofthe specimens was known. From each reading, the transit time for theapplied signal to return from the opposite surface of the material wasdetermined and the sonic velocity (V) was calculated according toequation 5.

$\begin{matrix}{V = {2 \times \frac{thickness}{{transit}\mspace{14mu} {time}}}} & (5)\end{matrix}$

3.4 Flexural Modulus Variation

Flexural modulus of a composite laminate is not usually considered as afundamental parameter of the material. It does, however, relate to howwell the layers are bonded to each other and the reinforcement is bondedto the resin. These properties are generally of importance to industrialusers, especially for fluid containment and support of bending moments.

FIG. 8 illustrates two feature zones within the A-Scan from FIG. 7. Fromthe Opposite Surface Feature Zone, the x-y plot data is used tocalculate the overall reflected ultrasound from the full structuralthickness of the composite. The x-y plot data from the IntermediateFeature Zone is processed and the results are incorporated with theopposite surface result to determine a value related to the overallattenuation of the applied signal. The value that is calculated has beengiven the variable name L and a functional description is given inEquation 2.

L=f(attenuation)  (2)

Calculation of L may be determined by the analysis module 125 byprocessing of the raw data from the ultrasonic instrument. After L isdetermined, it is formed into the parameter L_(tt), according toequation 4. The L_(tt) may be determined using data directly from theultrasonic readings and is intended not to require any additionalcalibrations or measurements.

L _(tt) =L×transit time  (4)

The L_(tt) may be determined from the A-Scan of each reading recorded.For each specimen, the average value was calculated and the differencesbetween the individual values and the average were determined. Thedistribution of the differences was evaluated.

4.1 Sonic Velocity Results

The sonic velocity distribution among the samples that did not containsand filler is shown in FIG. 9. Note that for each sample, there is afairly narrow range of velocity values calculated. There are somesimilarities in the data.

The sonic velocity results for the specimens with sand filler are shownin FIG. 10. Note that the velocity values for this case are lower thanin FIG. 9. For the samples shown, the mean is 2.29 mm/μs and thestandard deviation is 0.047 mm/μs.

FIG. 11 illustrates a histogram of all sonic velocity values for thesamples with no sand fill lumped together with an imposed normal curvewith a mean of 3.14 mm/μs and standard deviation of 0.149 mm/μs. Whenthe calculated probabilities are compared to the histogram fractionsthey are from the same population to a significance of 82%. A similarcurve with different mean and standard deviation for the sand-filledspecimens yields ANOVA significance of 81%.

For most new composites, the thickness is generally expected to bewithin a fairly narrow range, so ultrasonic readings should be expectedto give a narrow range of transit time values as calculated using thevelocity range shown in FIG. 11. In FIG. 11, the 95% confidence intervalof sonic velocity corresponds to ±9.3% of the mean. This means that FRCwith constant thickness will yield ultrasonic transit times with acorresponding confidence interval.

An appropriate quality specification for FRC would be to require thatthe confidence interval for transit time be within the confidenceinterval identified above and factored by the allowable thickness range.Unseen fillers and porosity are expected to reduce the sonic velocitywhich would be directly observable from transit time results. From this,unseen defects in a FRC laminate can be detected. Sonic velocity of FRChas been observed to decline from some service conditions. After thesonic velocity is determined for a new sample, a quality parameter canbe reported with the knowledge that the quality parameter may provideguidance for future inspections.

4.2 L_(tt) Variation

As discussed above the variation of L_(tt) values calculated for eachreading was determined. FIG. 12 is a histogram of distribution of thedifferences and the cumulative fraction. From the histogram, it may benoted that there is variation from the mean in almost all readings. Theresults illustrate that the variation from the mean is confined to anarrow band. The actual data was found to conform well to a normaldistribution with mean of 0 and standard deviation of 0.42, making the95% confidence limits ±0.81 with a nominal average L_(tt) of 3.50.

The average L_(tt) value from a new composite will also serve as astarting point to allow the progression of damage to be measured andevaluated while the FRC component is in service. It has been shown thatthe value of L_(tt) correlates directly to the flexural modulus ofFRC—thereby indicating changes in the inter-layer and fiber-matrixbonding. A close correlation of L_(tt) to tensile modulus has also beenshown.

A small value for standard deviation of L_(tt) is expected to relate tolower variation in the FRC and therefore more consistency in properties.An appropriate quality specification would be to require that 95% of allL_(tt) values for a new FRC be within the 95% confidence limits. Throughthe experiments, it has been determined that in a number of cases thatdefects in FRC will generally reduce the L_(tt) value to outside of theconfidence limits. Thus allowing detection of defects.

L_(tt) of FRC has been observed to decline due to service conditionssuch as high stress and corrosion. After the L_(tt) is determined for anew sample, the corresponding structural capacity can be determined andthis can be used in future inspections to determine structural changestaking place in the FRC. Both velocity and L_(tt) values may becalculated from one ultrasonic reading.

From the experiments, it was determined that 30 ultrasonic readings fromFRC that is expected to be constant thickness is expected to produce avalid mean and standard deviation. For 30 readings, adding one readingis expected to make less than 3% difference to the standard deviationfrom the value determined for 29 readings. Therefore an ultrasonicquality assessment may be completed with approximately 30 readings.

In a further experiment, 56 test specimens were examined. Of thesespecimens 41 were composed of polyester/vinyl ester resin and glasswhile 15 also includes various weight percentage of sand. In thisexperiment, the focus was on two particular parameters, namely the sonicvelocity and the flexural parameter.

$\begin{matrix}{V = {2 \times \frac{thickness}{{transit}\mspace{14mu} {time}}}} & (5) \\{L_{tt} = {L \times {transit}\mspace{14mu} {time}}} & (4)\end{matrix}$

FIGS. 13A and 13B illustrate the sonic velocity with no sand additionand the distribution with no sand. In particular, it can be viewed thatthere are overlapping ranges of the velocity range and the distributionillustrates a normal to 0.82 significance measure.

FIGS. 14A and 14B illustrate the sonic velocity with the sand includedin the matrix. From FIG. 14A, it can be seen that there is a widevariation between the samples. The distribution shown in FIG. 14B alsoillustrates that the variation from sand is significant and that thedistribution is normal to 0.21 significance.

FIGS. 15A and 15B illustrate the flexural parameter results. Theseresults may be calculated after each reading and may use only data fromthe ultrasonic data as no external information is required. The flexuralparameter results are intended to predict local and overall structuralproperties and pinpoint potential defects. Small differences to the meanindicate consistency. FIG. 9A is a histogram illustrating differences tothe mean and illustrates that variations beyond −0.83 may be a defect.FIG. 9B further illustrates defect detection.

A full set of ultrasonic readings to assess FRC of the same thicknesscan be obtained with approximately 30 or more ultrasonic readings usingthe method described herein. The sonic velocity of a composite is beexpected to be within a narrow range. The transit time for an ultrasonicsignal though FRC has been found to be directly proportional to thethickness and sonic velocity. Based on the expected range for sonicvelocity as shown above, and the thickness tolerance, the transit timefor an ultrasound pulse to transit the material can be specified to bewithin a known range.

In addition, the existence of defects and fillers in the FRC can bedetected from transit time variations that result from sonic velocityvariations. L_(tt) variation from the mean should be expected to bewithin a defined range for the composite which is intended to be lessthan approximately ±0.82 and should be expected to conform well to anormal distribution with mean of 0 and standard deviation of 0.42. Theexistence of defects in the FRC may be detected from L_(tt) variations.The existence of significant defects may reduce the calculated L_(tt)value at the defect. The values for sonic velocity and L_(tt) determinedfor new FRP may be used to determine changes taking place in serviceconditions.

Case studies have also been conducted using the method and systemdescribed herein. In a first case study, FRC pipes with 457.2 mm (18″)length and diameters ranging from 101.6 mm (4″) to 203.2 mm (8″) wereused. The FRC pipes were manufactured from glass fiber and epoxy vinylester using filament winding process. These pipes were fabricated fromthree distinct layers. The first layer is a liner, a smooth layer withspecial additives adapted to come into direct contact with the fluid.The main objective of this layer is to provide corrosion and wearresistance for the internal surface of the FRC pipe. The filament layeris the second layer that forms the pipe wall thickness and handles thestresses resulting from the internal and external pressure. Then a finallayer of pure resin coating is added to insure smooth surface finish andfull fiber impregnation. During the case study, information about thefiber orientation and the fiber weight percentage for the pipes receivedwas not provided. Prior knowledge of the fiber weight fraction, plythickness, and orientation has been considered to be necessary to obtainreliable ultrasonic testing prediction for the elastic modulus of theFRC pipes. Hence, burn off testing, ply counting and micrographs wereperformed to obtain this information. Three samples were extracted outthe circumferential direction of each pipe and placed in the oven under500° C. for an hour. The samples were weighed before and after the burnoff process and the fiber weight fraction (W_(f)) was calculated usingthe following relation,

$\begin{matrix}{W_{f{({Matrix})}} = \left( \frac{m_{c} - m_{f}}{m_{c}} \right)} & (8) \\{W_{f{({Fiber})}} = {1 - W_{f{({Matrix})}}}} & (9)\end{matrix}$

where m_(c) is the total mass of the composite before burn off and m_(f)is the mass of the remaining constituent after burn off. After burn offthe number of the plies was counted and stereomicroscope was used todetermine the fiber orientation as shown in FIG. 16. Table 4 shows theburn off and plies counting results.

TABLE 4 FRC pipe burn-off and ply counting results Calculated AverageFiber Fiber Pipe volume Weight diameter fraction fraction StandardNumber of (mm) (%) (%) Deviation Plies 101.6 (4″) 34.6 50.8 ±0.8 3 ply(57°) 3 ply (57°) 4 ply (Random) 152.4 (6″) 40.3 56.9 ±0.4 3 ply (57°) 3ply (57°) 4 ply (Random) 203.2 (8″) 39.8 56.4 ±0.7 3 ply (57°) 3 ply(57°) 4 ply (Random)

The weight fraction values can be converted to volume fraction using thefollowing relation,

$\begin{matrix}{V_{f{({fiber})}} = \frac{1}{{1 + {\frac{\rho_{f}}{\rho_{m}}\left( {\frac{1}{w_{f{({fiber})}}} - 1} \right)}}}} & (10)\end{matrix}$

where ρ_(m) is the matrix density and ρ_(f) is the fiber density. Theresulted data were used in the theoretical and experimental calculation.The FRC pipes were fabricated from a total of 10 plies. The first 6plies were stacked in [−57/+57]₆ configuration followed by 4 plies ofChopped Strand Mats (CSM) with random orientation. The orientation anglevalues for the unidirectional laminates obtained experimentally has adifference of 2.5° when compared to the reported value by themanufacturer (i.e. −54.5/+54.5). At orientation angles greater than 45°,a minor change in the orientation angle (i.e. 2.5°) is not expected tohave a significant effect on the tensile modulus.

Theoretical Evaluation for Elastic Properties of FRC Pipes

Micro-mechanical model is intended to predict the composite stiffnessproperties from then properties of its original constituents, listed inTable 5.

TABLE 5 FRC pipe constituent properties Tensile Tensile Density DiameterModulus Strength Poisson's Material (g/cm³) (μm) (GPa) (GPa) Ratio GlassFiber 2.54 10 (round) 72.4 3.45 0.2 (E-glass) Epoxy Vinyl 1.3 — 3.2 0.860.35 Ester

The elastic properties for unidirectional continuous fiber can becalculated using the Rule of Mixture (RoM)

$\begin{matrix}{E_{11} = {{E_{f}\left( {1 - V_{m}} \right)} + {E_{m}V_{m}}}} & (11) \\{E_{22} = \frac{E_{f}E_{m}}{{E_{f}V_{m}} + {E_{m}\left( {1 - V_{m}} \right)}}} & (12)\end{matrix}$

where E_(f) is the fiber modulus, E_(m) is the matrix modulus, V_(m) isthe matrix volume fraction. However, in this study a modified version ofthe rule of mixture, was used to account for the change in the fiberangle. In order to determine the micro-mechanical properties for the CSMwith random orientation, the following equation was used

E _(CSM)=⅜E _(xx)+⅝E _(yy)  (13)

where E_(xx) and E_(yy) are the longitudinal and transverse tensilemodulus for chopped unidirectional fibers. In this case the propertiesare assumed to be the same in all directions in the plane of the lamina(i.e. isotropic).

The elements in the stiffness matrix for an angle ply lamina [Q _(ij) ]were determined and then the extensional stiffness matrix and bendingstiffness matrix for the laminate was calculated using the CompositeLamination Theory (CLT). For simplicity, the sample assumed to be freefrom any geometrical curvatures and a perfect interlaminar bond existsbetween various laminas. The extensional stiffness matrix [A] and thebending matrix [D] were calculated using the following relations,

$\begin{matrix}{A_{ij} = {\sum\limits_{j = 1}^{N}{\left( {\overset{\_}{Q}}_{ij} \right)\left( {h_{j} - h_{j - 1}} \right)}}} & (14)\end{matrix}$

where N is the total number of laminas in the laminate, Q _(ij) ,elements of the stiffness matrix of the j^(th) lamina, and h_(j-1) isthe distance from the mid-plane to the top of the j^(th) lamina. The [A]matrix for the FRC pipes can be presented in a matrix form as:

$\begin{matrix}{\lbrack A\rbrack = \begin{bmatrix}A_{11} & A_{12} & A_{13} \\A_{21} & A_{22} & A_{23} \\A_{31} & A_{32} & A_{33}\end{bmatrix}} & (15)\end{matrix}$

Table 6 shows the theoretical elastic modulus values obtained using theCLT. It should be noticed that an increase of 4.5% in the calculatedtheoretical modulus values for the 4″ pipe was observed when compared tothe value of 8″ FRC; this is attributed to the increase of thefabricated FRP pipe wall thickness.

TABLE 6 Calculated theoretical properties of the FRC pipes Pipe TotalTensile Shear Diameter Thickness Modulus Modulus (mm) (mm) (GPa)) (GPa)101.6 (4″) 5.18 8.04 4.43 152.4 (6″) 6.04 8.24 5.07 203.2 (8″) 7.09 8.425.72

In order to obtain a systemic process for measuring and distinguishingthe data for each of the given points, a template of a grid system wasdeveloped and applied to the provided pipes. The grid system consists ofthe x-axis (identified by numbers) and y-axis (identified by alphabets)with the origin at the top right. 38.1 mm (1.5″) distance between thegrid points in both of the circumferential and the longitudinaldirection was adapted. Different pipe diameters will result inadditional data points of scan in the circumferential direction (i.e.hoop direction) of the pipe. A total of 120, 168 and 228 point werescanned for pipe diameters of 101.6 mm (4″), 152.4 (6″) and 203.2 mm(8″) respectively. Pulse-echo ultrasonic system was used to scan the FRCpipes across the pipe hoop direction. Low frequency flat transducer, 0.5MHz, with an element diameter of 32 mm (1″) coupled with a zero degreevulcanized rubber delay line were used. Readings were taken by holdingthe transducer in place on the pipe surface and then saving the readinginto the memory component of the ultrasonic system. After all readingswere taken, the raw reading data was extracted from the instrument. Theultrasonic data was then processed and analyzed by the system describedherein which calculates the ratio of expected actual modulus to thetheoretical modulus from CLT calculations. The ratio is known as thePercentage of Design Strength (PDS). Mathematically, PDS is expressed as

$\begin{matrix}{{PDS} = \frac{{Actual}\mspace{14mu} {Tensile}\mspace{14mu} {Modulus}}{{Theoretical}\mspace{14mu} {Tensile}\mspace{14mu} {Modulus}}} & (16)\end{matrix}$

The ultrasonic testing readings that were taken in the scans of thepipes were combined with pipe ID location, and assembled into a datafile that could be processed remotely. The raw data was scaled,normalized and filtered to account for variables in the collectionprocess such as transducer characteristics, pipe geometry and surfaceconditions and environmental conditions. The original A-scan wastransformed to allow quantitative analysis. An example of thistransformation is shown in FIG. 17. After transformation, patternrecognition was conducted to extract data from the signal reflectionsfrom the opposite surface, interfaces or defects within the laminate.The shape of various reflections and the time of their occurrence wereanalyzed and were used to calculate the shapes and relative magnitudesparameters of the identified signal that were used to determine anattenuation function (L). This attenuation function describes thedistribution and magnitude of the losses from the applied signal overthe signal path. The PDS was determined from L and the transit time orFRP thickness for the signal across the full pipe thickness. Table 7summarizes the results of the ultrasonic testing calculations.

TABLE 7 Developed ultrasonic method results summary PDS Calculated UsingSignal Transit Time PDS Calculated Using FRP Thickness PredictedPredicted Standard Hoop Standard Hoop Pipe Average Deviation ModulusAverage Deviation Modulus Diameter (%) (%) (GPa) (%) (%) (GPa) 101.6(4″) 76 ±13 6.11 79 ±14 6.35 152.4 (6″) 58 ±5 4.77 60 ±6 4.94 203.2 (8″)81 ±15 6.82 86 ±16 7.24

The PDS values can be applied to the theoretical modulus values fromequation (16) to determine the actual modulus. It was noticed that thepredicted modulus values obtained by the method described herein waslower than the theoretical calculated values (i.e. 19% differences forthe 8″ diameter) FRC. The theoretical calculation for the elasticproperties in Table 6 is based on the assumption that there is a perfectbonding between the composite constituents and the material is defectand void free. However, the ultrasonic testing predictions in Table 7account for discontinuities in materials, voids and the local variationin fiber-matrix bonding. Table 7 shows the corrected values for theultrasonic testing predicted modulus using the FRC pipe thickness, inpractice this correction criteria may be estimated as it may not bepossible to determine accurately. In order to measure the actual pipethickness, both side of the pipe should be accessible, which may not bethe case during field inspection.

A split ring test was conducted to determine the hoop modulus using(MTS-222 kN) hydraulic testing frame following test standard ASTM D2290,as shown in FIG. 19. In this test the FRC pipe was sliced to sectionswith a width of approximately 38 mm (1.5″) and then mounted on auniversal tensile machine using a special testing fixture. The ringswere subjected to loading and unloading cycle at a constant loading rateof 3 mm/min. Strain gages were mounted on the FRC pipes outer diameteralong the hoop direction, as shown in FIG. 18, to determine thecorresponding strain. The maximum load was kept below the yield strengthof the composite and the modulus was calculated from the linearregression analysis of the stress strain curve. An average of threerings per each pipe diameter were reported. All pipe rings were testedunder elastic deformation and no permanent deflection was applied to thesamples. Table 8 show the split ring testing values for two differentFRC pipes with diameter of 6″ and 8″. The average values for threedifferent rings for each FRC pipe diameter were reported. It can benoted from Table 8 that the ultrasonic analysis method described hereinhas the ability to predict the Hoop modulus accurately. Only adifference of 7.3% and 6.6% in the predicted modulus value was observedwhen compared to the experimental value obtained by the split ringtesting for FRP pipes with 6″ and 8″ diameter respectively.

TABLE 8 Comparison of Hoop tensile modules obtained by split ringtesting and Hoop tensile modules obtained by non-destructive testingPipe Spit Ring Hoop Predicted Hoop diameter Tensile Modulus TensileModulus (mm) (GPa) (GPa) 152.4 (6″) 5.15 ± 0.65 4.77 ± 0.41 203.2 (8″) 7.3 ± 0.46 6.82 ± 1.26

A further case study was performed wherein two experiments wereconducted in parallel to determine the effectiveness of this technology.In the first, ultrasonic readings were taken from the sample and then a3 point destructive test as per ASTM D790 was performed. In the second,the sample was initially preloaded and then ultrasonic readings weretaken followed by the ASTM D790 destructive test. The ultrasonicreadings were taken from an embodiment of the system described hereinthen analyzed by the method described herein.

Samples were separated into two parts each labeled A and B respectively.The following procedure was applied to both samples with the secondexperiment having an additional step of preloading before any ultrasonicmeasurements are captured. The preload was applied until the deflectionof the specimen reached 3% of the span.

1. Each of these parts was then further separated into 5 specimens witha width of 33+/−1 mm and a length which is equivalent to 16 times thethickness of the sample (as per ASTM D790 specification).2. A 25 mm by 40 mm section was cut from the sample which was used forignition loss testing to determine the theoretical modulus of thesample.3. Ultrasonic readings are taken at 32 mm intervals across the entirelength of the specimen4. ASTM D790 destructive tests were performed on each specimen

The ultrasonic readings were analyzed and compared with the resultsobtained from the ASTM D790 destructive tests. The raw data from theASTM D790 test gives the load-deflection curve of the sample as well asthe samples after fracture is shown in FIGS. 19A and 19B and FIGS. 20Aand 20B. The modulus from the destructive tests is then calculated asfollows:

$\begin{matrix}{E_{B} = \frac{L^{3}m}{4\; {bd}^{3}}} & \lbrack 17\rbrack\end{matrix}$

where E_(B) is the modulus of elasticity, L, m, b and d are the lengthof the span, slope of the tangent to the initial straight-line portionof the load deflection curve, width of the beam and depth of the beamrespectively.

Table 9 outlines the average dimension of each sample. The results areshown in FIGS. 21A and 21B. It can be seen that the results ofnondestructive tests and predictions are generally in agreement.Majority of the errors is within +/−20% as seen from FIGS. 21A and 21B.There are two results generated by the system for analyzing fiberreinforced composites, the first being Predicted Modulus for TransitTime, which calculates the modulus without knowledge or measurement ofthe samples thickness and the second being Predicted Modulus forThickness which takes into account the measured thickness of thematerial.

TABLE 9 Comparison between destructive and non-destructive tests SamplesWidth (m) Thickness (m) Span (m) Fibre Type N11A 0.0303 0.006 0.0978Long Fibres N11B 0.0303 0.006 0.0978 Long Fibres TH149161A 0.0302 0.00630.0978 Long Fibres TH149161B 0.0302 0.0063 0.0978 Long Fibres TW105031A0.0303 0.0126 0.163 Long Fibres TW105031B 0.0295 0.0113 0.163 LongFibres TW121352A 0.0298 0.0063 0.0978 Long Fibres TW121352B 0.02980.0063 0.0978 Long Fibres TW149163A 0.0303 0.0067 0.0978 Long FibresTW149163B 0.0303 0.0067 0.0978 Long Fibres YU15A 0.0356 0.0268 0.4018Long Fibres YU15B 0.0355 0.0263 0.4018 Long Fibres YU16A 0.0352 0.02610.4018 Long Fibres YU16B 0.0352 0.0276 0.4018 Long Fibres YU17A 0.03560.0278 0.4018 Long Fibres YU17B 0.0347 0.0253 0.4018 Long Fibres YU18A0.0355 0.0268 0.4018 Long Fibres YU18B 0.0352 0.026 0.4018 Long FibresYU19A 0.0356 0.0282 0.4018 Long Fibres YU19B 0.0363 0.0258 0.4018 LongFibres YU20B 0.0359 0.027 0.4018 Long Fibres YU22A 0.0386 0.008 0.0978Long Fibres YU22B 0.0389 0.0083 0.0978 Long Fibres YU23A 0.039 0.0050.0978 Long Fibres YU23B 0.0378 0.0053 0.0978 Long Fibres YU24A 0.03830.0082 0.0978 Long Fibres YU24B 0.0389 0.0082 0.0978 Long Fibres YU26A0.0378 0.0042 0.0978 Long Fibres YU26B 0.0385 0.0043 0.0978 Long FibresYU27A 0.038 0.0035 0.0978 Long Fibres YU27B 0.0388 0.0037 0.0978 LongFibres YU28A 0.0375 0.0056 0.0978 Long Fibres YU28B 0.0377 0.0051 0.0978Long Fibres YU29A 0.0381 0.006 0.0978 Long Fibres YU29B 0.0379 0.0060.0978 Long Fibres YU30A 0.038 0.0054 0.0978 Long Fibres YU30B 0.03780.0056 0.0978 Long Fibres YUS5A 0.0365 0.0092 0.163 Long Fibres YUS5B0.0362 0.0095 0.163 Long Fibres YUS3A 0.037 0.0105 0.163 Chopped FibresYUS4A 0.0383 0.0131 0.163 Long Fibres YUS7A 0.0363 0.0105 0.163 ChoppedFibres YUS7B 0.0361 0.0101 0.163 Chopped Fibres YUS1A 0.0351 0.00870.0978 Chopped Fibres YUS1B 0.0354 0.0089 0.0978 Chopped Fibres YUS2A0.0357 0.0088 0.155 Chopped Fibres YUS2B 0.0357 0.0089 0.155 ChoppedFibres

The residual service life of the sample can be estimated if thetheoretical modulus is known along with the time which the equipment hasbeen in operation. By plotting the modulus versus time, the residualservice life can be interpolated by a linear function. The following isan example calculation for sample #38. The theoretical modulus for thisspecimen was obtained using standard ignition test loss. This value isassumed to be the initial modulus at time zero. In practice, data sheetsmay be available to give more accurate information about the equipmentitself. It is also assumed that the specimen was in service for 5 years.

The modulus of sample #38 was estimated to be 4.1 GPa and thetheoretical modulus obtained from the ignition loss test is 5.53 GPa. Byexpressing both values as a percent of the initial value, the change inmodulus results in a decrease of approximately 5.2%/year. Therefore inanother 15 years the modulus will become 20% of its initial value whichcan be considered end of life.

The results from the nondestructive tests were found to be correlatedwith the prediction by the nondestructive test. Thus, the ultrasonicnondestructive test is intended to be a good indication of the modulusof material and the residual service life can then be inferred withknowledge of the design/theoretical modulus of the material. Also, it isnoted that the thickness of the material need not be known to obtainaccurate results. The thickness is inferred from the pulse transit time,which is intended to be another advantage of this method detailedherein. In practice, this may be considered an advantage for theultrasonic nondestructive test method as an inspector can inspect thepressure vessels from outside without the need to shut down the plant toget the thickness information by drilling.

FRC pipes are on the rise due to their superior corrosion resistance,lightweight and high strength-to-weight ratio, which makes themattractive to be used in many industrial product applications. Priorknowledge of FRC piping defects, methods of inspection and FRC pipematerial characterization prevents any unexpected failures. Thesedefects affect the structural integrity of FRC pipes and theirmechanical properties. Different methods and strategies for FRC pipeinspection can be used however embodiments of the ultrasonic testing andinspection method described herein are intended to provide advantagesover other method in terms of reliability, portability (in field use)and easiness. The method in intended to use easily available instrumentsalong with a short training cycle and highly automated data processing,thereby reducing inspector skill requirements.

In the preceding description, for purposes of explanation, numerousdetails are set forth in order to provide a thorough understanding ofthe embodiments. However, it will be apparent to one skilled in the artthat these specific details may not be required. In other instances,well-known structures are shown in block diagram form in order not toobscure the understanding. For example, specific details are notprovided as to whether the embodiments described herein are implementedas a software routine, hardware circuit, firmware, or a combinationthereof.

Embodiments of the disclosure or portions thereof can be represented asa computer program product stored in a machine-readable medium (alsoreferred to as a computer-readable medium, a processor-readable medium,or a computer usable medium having a computer-readable program codeembodied therein). The machine-readable medium can be any suitabletangible, non-transitory medium, including magnetic, optical, orelectrical storage medium including a diskette, compact disk read onlymemory (CD-ROM), memory device (volatile or non-volatile), or similarstorage mechanism. The machine-readable medium can contain various setsof instructions, code sequences, configuration information, or otherdata, which, when executed, cause a processor to perform steps in amethod according to an embodiment of the disclosure. Those of ordinaryskill in the art will appreciate that other instructions and operationsnecessary to implement the described implementations can also be storedon the machine-readable medium. The instructions stored on themachine-readable medium can be executed by a processor or other suitableprocessing device, and can interface with circuitry to perform thedescribed tasks.

The above-described embodiments are intended to be examples only.Alterations, modifications and variations can be effected to theparticular embodiments by those of skill in the art. The scope of thedescription should not be limited by the particular embodiments setforth herein, but should be construed in a manner consistent with thespecification as a whole.

Generally speaking, the description herein is intended to provide amethod for non-destructive analysis of fiber reinforced composites, themethod comprising: conducting ultrasonic analysis of the FRC;calculating a characteristic value based on the analysis; comparing thecharacteristic value based on the analysis to a baseline established forthe characteristic value; and determining a percentage of designstrength based on the comparison.

Generally speaking, there is also provided a system for non-destructiveanalysis of fiber reinforced composites (FRC), the system comprising: anultrasonic module for sending and receiving ultrasonic signals in theFRC and generating ultrasonic signal data; an analysis module forprocessing the ultrasonic signal data; a calculation module forcalculating a characteristic value based on the processing of theultrasonic signal data; a comparison module for comparing thecharacteristic value to a baseline established for the characteristicvalue; a determining module for determining a percentage of designstrength based on the comparison; and a processor for working with theother modules.

In some cases, the baseline for the characteristic value may bedetermined by linear regression analysis for a plurality of test resultsfor the characteristic value.

In some cases, the percentage of design strength result may be used todetermine a value for the remaining service life of the FRC.

We claim:
 1. A system for analyzing fiber reinforced composite, thesystem comprising: an ultrasonic transmitter configured to provideultra-sonic pulses to the fiber reinforced composite; an ultrasonicreceiver configured to receive ultrasonic signal data related to theultrasonic pulses; a filter module configured to filter the ultrasonicsignal data; a signal processing module configured to process thefiltered ultrasonic signal data; an analysis module configured toanalyze the processed ultrasonic signal data by: calculating acharacteristic value based on the ultrasonic signal data; comparing thecharacteristic value to a baseline established for the characteristicvalue; and determining a percentage of design strength based on thecomparison; and an output module configured to output the percentage ofdesign strength.
 2. A system according to claim 1, further comprising amemory component configured to store ultrasonic signal data and baselinefor characteristic values.
 3. A system according to claim 1, wherein thefilter module is configured to extract relevant data from the ultrasonicsignal data.
 4. A system according to claim 3, wherein the relevant datacomprises data associated with the material being tested and theextracted data includes data associated with the ultrasonic transmitterand the ultrasonic receiver.
 5. A system according to claim 3, whereinthe relevant data includes a magnitude of a reflection from the oppositesurface of the fiber reinforced composite.
 6. A system according toclaim 1, wherein the output module is further configured to output aprojection of a time remaining prior to a predetermined threshold valueis reached.
 7. A system according to claim 5 wherein the predeterminedthreshold value is a value related to a replacement requirement.
 8. Asystem of claim 1 wherein the output module is further configured tooutput data related to a strength level of a bonding at joins of thefiber reinforced composite.
 9. A method for analyzing fiberglassreinforced polymer, the method comprising: taking ultrasonic signal datafrom the fiber reinforced composite; receiving the ultrasonic signaldata, at an ultrasonic receiver; filtering the ultrasonic signal data,at a filter module; processing the filtered ultrasonic signal data, at asignal processing module; analyzing the processed ultrasonic signaldata, at an analysis module, wherein the analysis comprises: calculatinga characteristic value based on the ultrasonic signal data; comparingthe characteristic value to a baseline established for thecharacteristic value; and determining a percentage of design strengthbased on the comparison; and displaying the percentage of designstrength of the fiber reinforced composite, at an output module.
 10. Amethod according to claim 9, further comprising storing the ultrasonicsignal data and baseline for characteristic values at a memorycomponent.
 11. A method according to claim 9, wherein the filtering ofthe ultrasonic signal data comprises extracting relevant data from theultrasonic signal data.
 12. A method according to claim 11, wherein therelevant data comprises data associated with the material being testedand the extracted data includes data associated with the ultrasonictransmitter and the ultrasonic receiver.
 13. A method according to claim11, wherein the relevant data includes a magnitude of a reflection fromthe opposite surface of the fiber reinforced composite.
 14. A methodaccording to claim 9, further comprising displaying a projection of atime remaining prior to a predetermined threshold value is reached. 15.A method according to claim 14, wherein the predetermined thresholdvalue is a value related to a replacement requirement.
 16. A methodaccording to claim 9, further comprising displaying data related to astrength level of a bonding at joins of the fiber reinforced composite.